May 5, 2011

∴ Apple's Unused Drive Hybridization

New iMacs, released this past week, include a new controller chip (Cult of Mac) that could dramatically improve hard drive access time for little increased cost. Apple, however, chose to skip implementation of the chip's drive caching capability.

Computers have suffered from a number of speed bottlenecks over the years. One by one, they've faded. Moore's Law eventually eliminated the CPU as an impediment by cramming more transistors onto the chip. Cheap SDRAM prices have made plentiful memory common. The last bottleneck, hard drive access, fell as computer manufacturers began to adopt solid state drives (SSDs) for persistent storage, replacing traditional, spinning-platter hard drives. SSDs differ from processors and memory in one critical way, though: price.

CPUs and SDRAM have become cheaper over time, as have traditional hard drives, even as their speed and capacity greatly increased. SSDs are still fairly expensive. For example, upgrading a new iMac from a 1-terabyte hard drive to a 256-gigabyte SSD will set you back $500. The resulting machine will be blindingly fast, but will possess dramatically less storage for applications and data. While 256-megabytes is a plentiful amount of storage, for some users it isn't enough.

Hybrid drives are a good cure for this problem. By mating a small amount of SSD memory to a traditional hard drive, users get somewhat faster, still-cheap, and spacious storage. A controller chip onboard the drive makes the two units appear as one. It also controls which data gets copied to the SSD memory for quicker access. Seagate already sells such drives. Their performance is better than a hard drive alone, but not as quick as a full SSD. For that they'd need much larger SSD memory mated to the hard drive, or a way to logically combine a separate SSD and hard drive into one. The latter is exactly what the new Intel controller chip, built into new iMacs, can do.

The Intel Z68 controller chip can combine two separate drives, one a full SSD and the other a traditional hard drive, into a single volume. Importantly, it allows for the use of any SSD, and any hard drive. Manufacturers and users can select the right size/price trade-off. Since the SSD component is a full, multi-channel SSD in its own right, the performance gain can be great while the price bump reasonable, using a 40- or 80-gigabyte SSD. The user need never deal with two separate places to store programs and data.

Unfortunately, Apple appears to have chosen not to use this capability. New iMacs may be ordered with a hard drive, an SSD, or both. In the case of both, Apple indicates that the SSD will act as the boot drive, loaded with OS X and any pre-installed software. The hard drive ships empty. Without the drive caching capability, which goes unmentioned by Apple, the two drives appear as separate volumes and provide speed, or space, but not both at once.

It makes me wonder if Apple is going to make the new chip's caching capability a feature of future iMacs, perhaps as a technical refresh in a few months. What is clear is that by enabling the Z68's caching capability, an inexpensive, smaller SSD can bring significant improvement to otherwise average hard drive performance, killing the last speed bottleneck while not breaking the customer's budget.